> Okay, all you polymaths, here's a question that's got me stumped: why are so many substances brown?

Note that it is organic chemicals that tend to become brown. I suppose
this has to do with their tendency to absorb high energy photons and
convert them into low energy photons perhaps due to their structure
(lots of vibrational degrees of freedom sounds like a good reason for
being able to turn high energy photons into low energy photons,
acquiring vibrational momentum).